A vehicle engine is typically provided with a starter motor for starting the engine and a generator for generating electricity once the engine has started, and the accessories of the engine can be made compact and light in weight if these two pieces of equipment are combined into a single unit. Based on such a view point, various proposals have been made to use an electric motor also as a generator. Such a starter/generator is, not exclusively, suitable for use in small motorcycles, and typically consists of an electric rotating machine using permanent magnets for forming a magnetic field.
In such a starter/generator, it is necessary to generate as strong a magnetic field as possible for the given condition (size, weight, etc.) to the end of maximizing the output torque as a starter motor for starting the engine, but a strong magnetic field also results in an excessive generation of electricity when it is used as a generator. Therefore, the applicant previously proposed in Japanese patent laid open publication No. 07-59314 to provide a field control coil and control the strength of the magnetic field passing through the magnet poles. The magnetic field produced by the field control coil partly cancels the magnetic field generated by the permanent magnets, and controls the generation of electricity when it is used as a generator.
However, the permanent magnets and field control coil are arranged in different locations, and a desired control of the magnetic field is not always possible. Thus, it has not been possible to achieve both the maximization of output torque when used as an electric motor and the controlled generation of electricity when used as an electric generator without involving an unacceptable compromise.